The present invention relates to electrical distribution networks of the compensated type, that is to say that they comprise a compensation coil or Petersen coil which makes it possible to compensate for the capacitive current due to capacitances distributed in the line; the compensation coil makes it possible to obtain balance, that is to say perfect compensation for the capacitive impedances of the line.
The subject of the invention is, more particularly, a method and a device making it possible to measure the detuning of the compensation of the network; in fact, when the intrinsic capacitive impedance of the network varies, as a result, for example, of connecting or disconnecting high-load-factor consumers, the compensation coil is detuned, that is to say that it no longer exactly compensates for the overall capacitance of the network. In such a case it is advantageous to vary the value of the impedance of the compensation coil so as to reestablish balance.
It has already been proposed, in order to detect and measure the imbalance in the compensation of a network, to inject an electric current into the neutral circuit, the frequency of which current is slightly different from the frequency of the network; as a result of the appearance of a beat phenomenon, it is then possible to detect and measure the imbalance of the network. This method is relatively complex and requires a significant amount of equipment, in particular a generator having a different frequency from that of the network. Furthermore, this type of operation requires long adjustment and measurement periods.
It has also been proposed, as described in European Patent 235,145, to provide a device which makes it possible to add a supplementary reactance between the neutral and the earth of the network and subsequently to calculate the attenuation, asymmetry and imbalance parameters of the network. Connecting a supplementary reactance between the neutral and the earth can be done by impedance switching in parallel or in series with the compensation coil or even, in the case when the network comprises a variable compensation coil, by acting on the compensation coil itself.
This method has the drawback that the response time of the measurement thus carried out is high, and may be of the order of several minutes; the accuracy of the measurement is not very good, especially if the asymmetry is low, and this method cannot function reliably under all operational conditions of a network.